Method and system for design of enhanced accuracy patterns for charged particle beam lithography

ABSTRACT

A method and system for fracturing or mask data preparation are presented in which overlapping shots are generated to increase dosage in selected portions of a pattern, thus improving the fidelity and/or the critical dimension variation of the transferred pattern. In various embodiments, the improvements may affect the ends of paths or lines, or square or nearly-square patterns. Simulation is used to determine the pattern that will be produced on the surface.

RELATED APPLICATIONS

This application: 1) is a continuation-in-part of U.S. patentapplication Ser. No. 13/037,268 filed on Feb. 28, 2011, entitled “MethodAnd System For Design Of Enhanced Accuracy Patterns For Charged ParticleBeam Lithography”; and 2) is a continuation-in-part of U.S. patentapplication Ser. No. 13/650,618 filed on Oct. 12, 2012, entitled “MethodAnd System For Design Of A Reticle To Be Manufactured Using VariableShaped Beam Lithography”; both of which are hereby incorporated byreference for all purposes. U.S. patent application Ser. No. 13/650,618:3) is a continuation of U.S. patent application Ser. No. 13/316,564filed on Dec. 12, 2011 entitled “Method And System For Design Of AReticle To Be Manufactured Using Variable Shaped Beam Lithography” andissued as U.S. Pat. No. 8,304,148; 4) which is a continuation of U.S.patent application Ser. No. 13/087,334 filed on Apr. 14, 2011 entitled“Method and System For Design of a Reticle To Be Manufactured UsingVariable Shaped Beam Lithography” and issued as U.S. Pat. No. 8,202,672;5) which is a continuation of U.S. patent application Ser. No.12/987,994 filed on Jan. 10, 2011 entitled “Method For Manufacturing aSurface and Integrated Circuit Using Variable Shaped Beam Lithography”and issued as U.S. Pat. No. 8,017,289; 6) which is a continuation ofU.S. patent application Ser. No. 12/473,265 filed on May 27, 2009entitled “Method and System for Design of a Reticle to Be ManufacturedUsing Variable Shaped Beam Lithography” and issued as U.S. Pat. No.7,901,850; all of which are hereby incorporated by reference for allpurposes. U.S. Pat. No. 7,901,850: 7) is a continuation-in-part of U.S.patent application Ser. No. 12/202,366 filed Sep. 1, 2008, entitled“Method and System For Design of a Reticle to Be Manufactured UsingCharacter Projection Lithography” and issued as Ser. No. 7,759,027; and8) claims priority to U.S. Provisional Patent Application Ser. No.61/172,659, filed on Apr. 24, 2009 and entitled “Method forManufacturing a Surface and Integrated Circuit Using Variable ShapedBeam Lithography”; both of which are hereby incorporated by referencefor all purposes. This application also: 9) is related to Fujimura, U.S.patent application Ser. No. 13/037,263 filed on Feb. 28, 2011, entitled“Method and System For Design Of A Surface To Be Manufactured UsingCharged Particle Beam Lithography”; and 10) is related to Fujimura, U.S.patent application Ser. No. 13/037,270 filed on Feb. 28, 2011, entitled“Method and System For Design Of Enhanced Edge Slope Patterns ForCharged Particle Beam Lithography”; both of which are herebyincorporated by reference for all purposes.

BACKGROUND OF THE DISCLOSURE

The present disclosure is related to lithography, and more particularlyto the design and manufacture of a surface which may be a reticle, awafer, or any other surface, using charged particle beam lithography.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, optical lithography may be used to fabricate thesemiconductor devices. Optical lithography is a printing process inwhich a lithographic mask or photomask manufactured from a reticle isused to transfer patterns to a substrate such as a semiconductor orsilicon wafer to create the integrated circuit (I.C.). Other substratescould include flat panel displays or even other reticles. Conventionaloptical lithography typically uses radiation of 193 nm wavelength orlonger. Extreme ultraviolet (EUV) or X-ray lithography are alsoconsidered types of optical lithography, but use wavelengths muchshorter than the 193 nm of conventional optical lithography. The reticleor multiple reticles may contain a circuit pattern corresponding to anindividual layer of the integrated circuit, and this pattern can beimaged onto a certain area on the substrate that has been coated with alayer of radiation-sensitive material known as photoresist or resist.Once the patterned layer is transferred the layer may undergo variousother processes such as etching, ion-implantation (doping),metallization, oxidation, and polishing. These processes are employed tofinish an individual layer in the substrate. If several layers arerequired, then the whole process or variations thereof will be repeatedfor each new layer. Eventually, a combination of multiples of devices orintegrated circuits will be present on the substrate. These integratedcircuits may then be separated from one another by dicing or sawing andthen may be mounted into individual packages. In the more general case,the patterns on the substrate may be used to define artifacts such asdisplay pixels, holograms, or magnetic recording heads.

In the production or manufacturing of semiconductor devices, such asintegrated circuits, maskless direct write may also be used to fabricatethe semiconductor devices. Maskless direct write is a printing processin which charged particle beam lithography is used to transfer patternsto a substrate such as a semiconductor or silicon wafer to create theintegrated circuit. Other substrates could include flat panel displays,imprint masks for nano-imprinting, or even reticles. Desired patterns ofa layer are written directly on the surface, which in this case is alsothe substrate. Once the patterned layer is transferred the layer mayundergo various other processes such as etching, ion-implantation(doping), metallization, oxidation, and polishing. These processes areemployed to finish an individual layer in the substrate. If severallayers are required, then the whole process or variations thereof willbe repeated for each new layer. Some of the layers may be written usingoptical lithography while others may be written using maskless directwrite to fabricate the same substrate. Also, some patterns of a givenlayer may be written using optical lithography, and other patternswritten using maskless direct write. Eventually, a combination ofmultiples of devices or integrated circuits will be present on thesubstrate. These integrated circuits are then separated from one anotherby dicing or sawing and then mounted into individual packages. In themore general case, the patterns on the surface may be used to defineartifacts such as display pixels, holograms, or magnetic recordingheads.

Two common types of charged particle beam lithography are variableshaped beam (VSB) and character projection (CP). These are bothsub-categories of shaped beam charged particle beam lithography, inwhich a precise electron beam is shaped and steered so as to expose aresist-coated surface, such as the surface of a wafer or the surface ofa reticle. In VSB, these shapes are simple shapes, usually limited torectangles of certain minimum and maximum sizes and with sides which areparallel to the axes of a Cartesian coordinate plane (i.e. of“manhattan” orientation), and 45 degree right triangles (i.e. triangleswith their three internal angles being 45 degrees, 45 degrees, and 90degrees) of certain minimum and maximum sizes. At predeterminedlocations, doses of electrons are shot into the resist with these simpleshapes. The total writing time for this type of system increases withthe number of shots. In character projection (CP), there is a stencil inthe system that has in it a variety of apertures or characters which maybe complex shapes such as rectilinear, arbitrary-angled linear,circular, nearly circular, annular, nearly annular, oval, nearly oval,partially circular, partially nearly circular, partially annular,partially nearly annular, partially nearly oval, or arbitrarycurvilinear shapes, and which may be a connected set of complex shapesor a group of disjointed sets of a connected set of complex shapes. Anelectron beam can be shot through a character on the stencil toefficiently produce more complex patterns on the reticle. In theory,such a system can be faster than a VSB system because it can shoot morecomplex shapes with each time-consuming shot. Thus, an E-shaped patternshot with a VSB system takes four shots, but the same E-shaped patterncan be shot with one shot with a character projection system. Note thatVSB systems can be thought of as a special (simple) case of characterprojection, where the characters are just simple characters, usuallyrectangles or 45-45-90 degree triangles. It is also possible topartially expose a character. This can be done by, for instance,blocking part of the particle beam. For example, the E-shaped patterndescribed above can be partially exposed as an F-shaped pattern or anI-shaped pattern, where different parts of the beam are cut off by anaperture. This is the same mechanism as how various sized rectangles canbe shot using VSB. In this disclosure, partial projection is used tomean both character projection and VSB projection.

As indicated, in optical lithography the lithographic mask or reticlecomprises geometric patterns corresponding to the circuit components tobe integrated onto a substrate. The patterns used to manufacture thereticle may be generated utilizing computer-aided design (CAD) softwareor programs. In designing the patterns the CAD program may follow a setof pre-determined design rules in order to create the reticle. Theserules are set by processing, design, and end-use limitations. An exampleof an end-use limitation is defining the geometry of a transistor in away in which it cannot sufficiently operate at the required supplyvoltage. In particular, design rules can define the space tolerancebetween circuit devices or interconnect lines. The design rules are, forexample, used to ensure that the circuit devices or lines do notinteract with one another in an undesirable manner. For example, thedesign rules are used so that lines do not get too close to each otherin a way that may cause a short circuit. The design rule limitationsreflect, among other things, the smallest dimensions that can bereliably fabricated. When referring to these small dimensions, oneusually introduces the concept of a critical dimension. These are, forinstance, defined as the smallest width of a line or the smallest spacebetween two lines, those dimensions requiring exquisite control.

One goal in integrated circuit fabrication by optical lithography is toreproduce the original circuit design on the substrate by use of thereticle. Integrated circuit fabricators are always attempting to use thesemiconductor wafer real estate as efficiently as possible. Engineerskeep shrinking the size of the circuits to allow the integrated circuitsto contain more circuit elements and to use less power. As the size ofan integrated circuit critical dimension is reduced and its circuitdensity increases, the critical dimension of the circuit pattern orphysical design approaches the resolution limit of the optical exposuretool used in conventional optical lithography. As the criticaldimensions of the circuit pattern become smaller and approach theresolution value of the exposure tool, the accurate transcription of thephysical design to the actual circuit pattern developed on the resistlayer becomes difficult. To further the use of optical lithography totransfer patterns having features that are smaller than the lightwavelength used in the optical lithography process, a process known asoptical proximity correction (OPC) has been developed. OPC alters thephysical design to compensate for distortions caused by effects such asoptical diffraction and the optical interaction of features withproximate features. OPC includes all resolution enhancement technologiesperformed with a reticle.

OPC may add sub-resolution lithographic features to mask patterns toreduce differences between the original physical design pattern, thatis, the design, and the final transferred circuit pattern on thesubstrate. The sub-resolution lithographic features interact with theoriginal patterns in the physical design and with each other andcompensate for proximity effects to improve the final transferredcircuit pattern. One feature that is used to improve the transfer of thepattern is a sub-resolution assist feature (SRAF). Another feature thatis added to improve pattern transference is referred to as “serifs”.Serifs are small features that can be positioned on a corner of apattern to sharpen the corner in the final transferred image. It isoften the case that the precision demanded of the surface manufacturingprocess for SRAFs are less than those for patterns that are intended toprint on the substrate, often referred to as main features. Serifs are apart of a main feature. As the limits of optical lithography are beingextended far into the sub-wavelength regime, the OPC features must bemade more and more complex in order to compensate for even more subtleinteractions and effects. As imaging systems are pushed closer to theirlimits, the ability to produce reticles with sufficiently fine OPCfeatures becomes critical. Although adding serifs or other OPC featuresto a mask pattern is advantageous, it also substantially increases thetotal feature count in the mask pattern. For example, adding a serif toeach of the corners of a square using conventional techniques adds eightmore rectangles to a mask or reticle pattern. Adding OPC features is avery laborious task, requires costly computation time, and results inmore expensive reticles. Not only are OPC patterns complex, but sinceoptical proximity effects are long range compared to minimum line andspace dimensions, the correct OPC patterns in a given location dependsignificantly on what other geometry is in the neighborhood. Thus, forinstance, a line end will have different size serifs depending on whatis near it on the reticle. This is even though the objective might be toproduce exactly the same shape on the wafer. These slight but criticalvariations are important and have prevented others from being able toform reticle patterns. It is conventional to discuss the OPC-decoratedpatterns to be written on a reticle in terms of main features, that isfeatures that reflect the design before OPC decoration, and OPCfeatures, where OPC features might include serifs, jogs, and SRAF. Toquantify what is meant by slight variations, a typical slight variationin OPC decoration from neighborhood to neighborhood might be 5% to 80%of a main feature size. Note that for clarity, variations in the designof the OPC are what is being referenced. Manufacturing variations, suchas line-edge roughness and corner rounding, will also be present in theactual surface patterns. When these OPC variations produce substantiallythe same patterns on the wafer, what is meant is that the geometry onthe wafer is targeted to be the same within a specified error, whichdepends on the details of the function that that geometry is designed toperform, e.g., a transistor or a wire. Nevertheless, typicalspecifications are in the 2%-50% of a main feature range. There arenumerous manufacturing factors that also cause variations, but the OPCcomponent of that overall error is often in the range listed. OPC shapessuch as sub-resolution assist features are subject to various designrules, such as a rule based on the size of the smallest feature that canbe transferred to the wafer using optical lithography. Other designrules may come from the mask manufacturing process or, if a characterprojection charged particle beam writing system is used to form thepattern on a reticle, from the stencil manufacturing process. It shouldalso be noted that the accuracy requirement of the SRAF features on themask may be lower than the accuracy requirements for the main featureson the mask.

There are a number of technologies used for forming patterns on areticle, including using optical lithography or charged particle beamlithography. The most commonly used system is the variable shaped beam(VSB), where, as described above, doses of electrons with simple shapessuch as manhattan rectangles and 45-degree right triangles expose aresist-coated reticle surface. In conventional mask writing, the dosesor shots of electrons are conventionally designed to avoid overlapwherever possible, so as to greatly simplify calculation of how theresist on the reticle will register the pattern. Similarly, the set ofshots is designed so as to completely cover the pattern area that is tobe formed on the reticle.

Reticle writing for the most advanced technology nodes typicallyinvolves multiple passes of charged particle beam writing, a processcalled multi-pass exposure, whereby the given shape on the reticle iswritten and overwritten. Typically, two to four passes are used to writea reticle to average out precision errors in the charged particle beamwriter, allowing the creation of more accurate photomasks. Alsotypically, the list of shots, including the dosages, is the same forevery pass. In one variation of multi-pass exposure, the lists of shotsmay vary among exposure passes, but the union of the shots in anyexposure pass covers the same area. Multi-pass writing can reduceover-heating of the resist coating the surface. Multi-pass writing alsoaverages out random errors of the charged particle beam writer.Multi-pass writing using different shot lists for different exposurepasses can also reduce the effects of certain systemic errors in thewriting process.

In EUV lithography, OPC features are generally not required. Therefore,the complexity of the pattern to be manufactured on the reticle is lessthan with conventional 193 nm wavelength optical lithography, and shotcount reduction is correspondingly less important. In EUV, however, maskaccuracy requirements are very high because the patterns on the mask,which are typically 4× the size of the patterns on the wafer, aresufficiently small that they are challenging to form precisely usingcharged particle beam technology such as E-beam.

There are numerous undesirable short-range and long-range effectsassociated with charged particle beam exposure. These effects can causedimensional inaccuracies in the pattern transferred to a surface such asa reticle. These effects can also increase the dimensional changes thatnormal process variations cause in the transferred pattern. It would bedesirable both to increase the accuracy of the transferred pattern, andalso to reduce the dimensional changes associated with processvariations.

SUMMARY OF THE DISCLOSURE

A method and system for fracturing or mask data preparation arepresented in which overlapping shots are generated to increase dosage inselected portions of a pattern, thus improving the fidelity and/or thecritical dimension variation of the transferred pattern. In variousembodiments, the improvements may affect the ends of paths or lines, orsquare or nearly-square patterns. The shots may be varied in theiramount of overlap, shot size, and dosage with respect to the dosage ofanother overlapping shot. Simulation is used to determine the patternthat will be produced on the surface. A method for manufacturing asurface is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a character projection charged particlebeam system;

FIG. 2A illustrates an example of a single charged particle beam shotand a cross-sectional dosage graph of the shot;

FIG. 2B illustrates an example of a pair of proximate shots and across-sectional dosage graph of the shot pair;

FIG. 2C illustrates an example of a pattern formed on a resist-coatedsurface from the pair of FIG. 2B shots;

FIG. 3A illustrates an example of a polygonal pattern;

FIG. 3B illustrates an example of a conventional fracturing of thepolygonal pattern of FIG. 3A;

FIG. 3C illustrates an example of an alternate fracturing of thepolygonal pattern of FIG. 3A;

FIG. 4A illustrates an example of a shot outline from a rectangularshot;

FIG. 4B illustrates an example of a longitudinal dosage curve for theshot of FIG. 4A using a normal shot dosage;

FIG. 4C illustrates an example of a longitudinal dosage curve similar toFIG. 4B, with long-range effects included;

FIG. 4D illustrates an example of a longitudinal dosage curve for theshot of FIG. 4A using a higher than normal shot dosage;

FIG. 4E illustrates an example of a longitudinal dosage curve similar toFIG. 4D, with long-range effects included;

FIG. 4F illustrates an example of a longitudinal dosage curve similar toFIG. 4E, but with a higher background dosage level;

FIG. 5A illustrates an example of how a 100 nm square VSB shot may beregistered on a reticle;

FIG. 5B illustrates an example of how a 60 nm square VSB shot may beregistered on a reticle;

FIG. 6A illustrates an example of a pattern comprising the end portionof a line;

FIG. 6B illustrates an example of a conventional single-shot method offorming the pattern of FIG. 6A on a surface;

FIG. 6C illustrates an example of a method of forming the pattern ofFIG. 6A on a surface by one embodiment of the current invention;

FIG. 6D illustrates an example of a method of forming the pattern ofFIG. 6A on a surface by another embodiment of the current invention;

FIG. 6E illustrates an example of a method of forming the pattern ofFIG. 6A on a surface by yet another embodiment of the current invention;

FIG. 7 illustrates a conceptual flow diagram of how to prepare asurface, such as a reticle, for use in fabricating a substrate such asan integrated circuit on a silicon wafer using optical lithography;

FIG. 8 illustrates a conceptual flow diagram of how to prepare a surfacefor use in fabricating a substrate such as an integrated circuit on asilicon wafer;

FIG. 9A illustrates a square pattern to be formed on a surface;

FIG. 9B illustrates a single-shot method of forming the pattern of FIG.9A on a surface;

FIG. 9C illustrates an example of a method of forming the pattern ofFIG. 9A on a surface by one embodiment of the current invention;

FIG. 9D illustrates an example of a method of forming the pattern ofFIG. 9A on a surface by another embodiment of the current invention; and

FIG. 9E illustrates an example of a method of forming the pattern ofFIG. 9A on a surface by yet another embodiment of the current invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes a method for fracturing patterns intoshots for a charged particle beam writer, where overlapping shots aregenerated to improve the accuracy and/or the edge slope of the patternwritten to a surface. The use of overlapping shots in this applicationtypically increases shot count and exposure time.

Referring now to the drawings, wherein like numbers refer to like items,FIG. 1 illustrates an embodiment of a conventional lithography system100, such as a charged particle beam writer system, in this case anelectron beam writer system, that employs character projection tomanufacture a surface 130. The electron beam writer system 100 has anelectron beam source 112 that projects an electron beam 114 toward anaperture plate 116. The plate 116 has an aperture 118 formed thereinwhich allows the electron beam 114 to pass. Once the electron beam 114passes through the aperture 118 it is directed or deflected by a systemof lenses (not shown) as electron beam 120 toward another rectangularaperture plate or stencil mask 122. The stencil 122 has formed therein anumber of openings or apertures 124 that define various types ofcharacters 126, which may be complex characters. Each character 126formed in the stencil 122 may be used to form a pattern 148 on a surface130 of a substrate 132, such as a silicon wafer, a reticle or othersubstrate. In partial exposure, partial projection, partial characterprojection, or variable character projection, electron beam 120 may bepositioned so as to strike or illuminate only a portion of one of thecharacters 126, thereby forming a pattern 148 that is a subset ofcharacter 126. For each character 126 that is smaller than the size ofthe electron beam 120 defined by aperture 118, a blanking area 136,containing no aperture, is designed to be adjacent to the character 126,so as to prevent the electron beam 120 from illuminating an unwantedcharacter on stencil 122. An electron beam 134 emerges from one of thecharacters 126 and passes through an electromagnetic or electrostaticreduction lens 138 which reduces the size of the pattern from thecharacter 126. In commonly available charged particle beam writersystems, the reduction factor is between 10 and 60. The reduced electronbeam 140 emerges from the reduction lens 138, and is directed by aseries of deflectors 142 onto the surface 130 as the pattern 148, whichis depicted as being in the shape of the letter “H” corresponding tocharacter 126A. The pattern 148 is reduced in size compared to thecharacter 126A because of the reduction lens 138. The pattern 148 isdrawn by using one shot of the electron beam system 100. This reducesthe overall writing time to complete the pattern 148 as compared tousing a variable shape beam (VSB) projection system or method. Althoughone aperture 118 is shown being formed in the plate 116, it is possiblethat there may be more than one aperture in the plate 116. Although twoplates 116 and 122 are shown in this example, there may be only oneplate or more than two plates, each plate comprising one or moreapertures.

In conventional charged particle beam writer systems the reduction lens138 is calibrated to provide a fixed reduction factor. The reductionlens 138 and/or the deflectors 142 also focus the beam on the plane ofthe surface 130. The size of the surface 130 may be significantly largerthan the maximum beam deflection capability of the deflection plates142. Because of this, patterns are normally written on the surface in aseries of stripes. Each stripe contains a plurality of sub-fields, wherea sub-field is within the beam deflection capability of the deflectionplates 142. The electron beam writer system 100 contains a positioningmechanism 150 to allow positioning the substrate 132 for each of thestripes and sub-fields. In one variation of the conventional chargedparticle beam writer system, the substrate 132 is held stationary whilea sub-field is exposed, after which the positioning mechanism 150 movesthe substrate 132 to the next sub-field position. In another variationof the conventional charged particle beam writer system, the substrate132 moves continuously during the writing process. In this variationinvolving continuous movement, in addition to deflection plates 142,there may be another set of deflection plates (not shown) to move thebeam at the same speed and direction as the substrate 132 is moved.

The minimum size pattern that can be projected with reasonable accuracyonto a surface 130 is limited by a variety of short-range physicaleffects associated with the electron beam writer system 100 and with thesurface 130, which normally comprises a resist coating on the substrate132. These effects include forward scattering, Coulomb effect, andresist diffusion. Beam blur is a term used to include all of theseshort-range effects. The most modern electron beam writer systems canachieve an effective beam blur in the range of 20 nm to 30 nm. Forwardscattering may constitute one quarter to one half of the total beamblur. Modern electron beam writer systems contain numerous mechanisms toreduce each of the constituent pieces of beam blur to a minimum. Someelectron beam writer systems may allow the beam blur to be varied duringthe writing process, from the minimum value available on an electronbeam writing system to one or more larger values.

The shot dosage of a charged particle beam writer such as an electronbeam writer system is a function of the intensity of the beam source 112and the exposure time for each shot. Typically the beam intensityremains fixed, and the exposure time is varied to obtain variable shotdosages. The exposure time may be varied to compensate for variouslong-range effects such as backscatter and fogging in a process calledproximity effect correction (PEC). Electron beam writer systems usuallyallow setting an overall dosage, called a base dosage, that affects allshots in an exposure pass. Some electron beam writer systems performdosage compensation calculations within the electron beam writer systemitself, and do not allow the dosage of each shot to be assignedindividually as part of the input shot list, the input shots thereforehaving unassigned shot dosages. In such electron beam writer systems allshots have the base dosage, before proximity effect correction. Otherelectron beam writer systems do allow dosage assignment on ashot-by-shot basis. In electron beam writer systems that allowshot-by-shot dosage assignment, the number of available dosage levelsmay be 64 to 4096 or more, or there may be a relatively few availabledosage levels, such as 3 to 8 levels. Some embodiments of the currentinvention are targeted for use with charged particle beam writingsystems which either do not allow dosage assignment on a shot-by-shotbasis, or which allow assignment of one of a relatively few dosagelevels.

FIGS. 2A-B illustrate how energy is registered on a resist-coatedsurface from one or more charged particle beam shots. In FIG. 2Arectangular pattern 202 illustrates a shot outline, which is a patternthat will be produced on a resist-coated surface from a shot which isnot proximate to other shots. The corners of pattern 202 are rounded dueto beam blur. In dosage graph 210, dosage curve 212 illustrates thecross-sectional dosage along a line 204 through shot outline 202. Line214 denotes the resist threshold, which is the dosage above which theresist will register a pattern. As can be seen from dosage graph 210,dosage curve 212 is above the resist threshold between the X-coordinates“a” and “b”. Coordinate “a” corresponds to dashed line 216, whichdenotes the left-most extent of the shot outline 202. Similarly,coordinate “b” corresponds to dashed line 218, which denotes theright-most extent of the shot outline 202. The shot dosage for the shotin the example of FIG. 2A is a normal dosage, as marked on dosage graph210. In conventional mask writing methodology, the normal dosage is setso that a relatively large rectangular shot will register a pattern ofthe desired size on the resist-coated surface, in the absence oflong-range effects. The normal dosage therefore depends on the value ofthe resist threshold 214.

FIG. 2B illustrates the shot outlines of two particle beam shots, andthe corresponding dosage curve. Shot outline 222 and shot outline 224result from two proximate particle beam shots. In dosage graph 220,dosage curve 230 illustrates the dosage along the line 226 through shotoutlines 222 and 224. As shown in dosage curve 230, the dosageregistered by the resist along line 226 is the combination, such as thesum, of the dosages from two particle beam shots, represented by shotoutline 222 and shot outline 224. As can be seen, dosage curve 230 isabove the threshold 214 from X-coordinate “a” to X-coordinate “d”. Thisindicates that the resist will register the two shots as a single shape,extending from coordinate “a” to coordinate “d”. FIG. 2C illustrates apattern 252 that the two shots from the example of FIG. 2B may form. Thevariable width of pattern 252 is the result of the gap between shotoutline 222 and shot outline 224, and illustrates that a gap between theshots 222 and 224 causes dosage to drop below threshold near the cornersof the shot outlines closest to the gap.

When using conventional non-overlapping shots using a single exposurepass, conventionally all shots are assigned a normal dosage before PECdosage adjustment. A charged particle beam writer which does not supportshot-by-shot dosage assignment can therefore be used by setting the basedosage to a normal dosage. If multiple exposure passes are used withsuch a charged particle beam writer, the base dosage is conventionallyset according to the following equation:

base dosage=normal dosage/# of exposure passes

FIGS. 3A-C illustrate two known methods of fracturing a polygonalpattern. FIG. 3A illustrates a polygonal pattern 302 that is desired tobe formed on a surface. FIG. 3B illustrates a conventional method offorming this pattern using non-overlapping or disjoint shots. Shotoutline 310, shot outline 312 and shot outline 314, which are markedwith X's for clarity, are mutually disjoint. Additionally, the threeshots associated with these shot outlines all use a desired normaldosage, before proximity effect correction. An advantage of using theconventional method as shown in FIG. 3B is that the response of theresist can be easily predicted. Also, the shots of FIG. 3B can beexposed using a charged particle beam system which does not allow dosageassignment on a shot-by-shot basis, by setting the base dosage of thecharged particle beam writer to the normal dosage. FIG. 3C illustratesan alternate method of forming the pattern 302 on a resist-coatedsurface using overlapping shots, disclosed in U.S. Pat. No. 7,901,850issued Mar. 8, 2011, and entitled “Method And System For Design Of AReticle To Be Manufactured Using Variable Shaped Beam Lithography.” InFIG. 3C the constraint that shot outlines cannot overlap has beeneliminated, and shot 320 and shot 322 do overlap. In the example of FIG.3C, allowing shot outlines to overlap enables forming the pattern 302 inonly two shots, compared to the three shots of FIG. 3B. In FIG. 3C,however the response of the resist to the overlapping shots is not aseasily predicted as in FIG. 3B. In particular, the interior corners 324,326, 328 and 330 may register as excessively rounded because of thelarge dosage received by overlapping region 332, shown by horizontalline shading. Charged particle beam simulation may be used to determinethe pattern registered by the resist. In one embodiment, chargedparticle beam simulation may be used to calculate the dosage for eachgrid location in a two-dimensional (X and Y) grid, creating a grid ofcalculated dosages called a dosage map. The results of charged particlebeam simulation may indicate use of non-normal dosages for shot 320 andshot 322. Additionally, in FIG. 3C the overlapping of shots in area 332increases the area dosage beyond what it would be without shot overlap.While the overlap of two individual shots will not increase the areadosage significantly, this technique will increase area dosages andtotal dosage if used throughout a design.

In exposing, for example, a repeated pattern on a surface using chargedparticle beam lithography, the size of each pattern instance, asmeasured on the final manufactured surface, will be slightly different,due to manufacturing variations. The amount of the size variation is anessential manufacturing optimization criterion. In mask masking today, aroot mean square (RMS) variation of no more than 1 nm (1 sigma) may bedesired. More size variation translates to more variation in circuitperformance, leading to higher design margins being required, making itincreasingly difficult to design faster, lower-power integratedcircuits. This variation is referred to as critical dimension (CD)variation. A low CD variation is desirable, and indicates thatmanufacturing variations will produce relatively small size variationson the final manufactured surface. In the smaller scale, the effects ofa high CD variation may be observed as line edge roughness (LER). LER iscaused by each part of a line edge being slightly differentlymanufactured, leading to some waviness in a line that is intended tohave a straight edge. CD variation is inversely related to the slope ofthe dosage curve at the resist threshold, which is called edge slope.Therefore, edge slope, or dose margin, is a critical optimization factorfor particle beam writing of surfaces.

FIG. 4A illustrates an example of an outline of a rectangular shot 402.FIG. 4B illustrates an example of a dosage graph 410 illustrating thedosage along the line 404 through shot outline 402 with a normal shotdosage, with no backscatter, such as would occur if shot 402 was theonly shot within the range of backscattering effect, which, as anexample, may be 10 microns. Other long-range effects are also assumed tocontribute nothing to the background exposure of FIG. 4B, leading to azero background exposure level. The total dosage delivered to the resistis illustrated on the y-axis, and is 100% of a normal dosage. Because ofthe zero background exposure, the total dosage and the shot dosage arethe same. Dosage graph 410 also illustrates the resist threshold 414.The CD variation of the shape represented by dosage graph 410 in thex-direction is inversely related to the slope of the dosage curve 412 atx-coordinates “a” and “b” where it intersects the resist threshold.

The FIG. 4B condition of zero background exposure is not reflective ofactual designs. Actual designs will typically have many other shotswithin the backscattering distance of shot 402. FIG. 4C illustrates anexample of a dosage graph 420 of a shot with a normal dosage withnon-zero background exposure 428. In this example, a background exposureof 20% of a normal dosage is shown. In dosage graph 420, dosage curve422 illustrates the cross-sectional dosage of a shot similar to shot402. The CD variation of curve 422 is worse than the CD variation ofcurve 412, as indicated by the lower edge slope where curve 422intersects resist threshold 424 at points “a” and “b”, due to thebackground exposure caused by backscatter.

One method of increasing the slope of the dosage curve at the resistthreshold is to increase the shot dosage. FIG. 4D illustrates an exampleof a dosage graph 430 with a dosage curve 432 which illustrates a totaldosage of 150% of normal dosage, with no background exposure. With nobackground exposure, the shot dosage equals the total dosage. Threshold434 in FIG. 4D is unchanged from threshold 414 in FIG. 4B. Increasingshot dosage increases the size of a pattern registered by the resist.Therefore, to maintain the size of the resist pattern, illustrated asthe intersection points of dosage curve 432 with threshold 434, the shotsize used for dosage graph 430 is somewhat smaller than shot 402. As canbe seen, the slope of dosage curve 432 is higher where it intersectsthreshold 434 than is the slope of dosage curve 412 where it intersectsthreshold 414, indicating a lower, improved, CD variation for thehigher-dosage shot of FIG. 4D than for the normal dosage shot of FIG.4B.

Like dosage graph 410, however, the zero background exposure conditionof dosage graph 430 is not reflective of actual designs. FIG. 4Eillustrates an example of a dosage graph 440 with the shot dosageadjusted to achieve a total dosage on the resist of 150% of normaldosage with a 20% background exposure, such as would occur if the dosageof only one shot was increased to achieve total dosage of 150% of anormal dosage, and dosage of other shots remained at 100% of normaldosage. The threshold 444 is the same as in FIGS. 4B-4D. The backgroundexposure is illustrated as line 448. As can be seen, the slopes ofdosage curve 442 at x-coordinates “a” and “b” are less than the slopesof dosage curve 432 at x-coordinates “a” and “b” because of the presenceof backscatter. Comparing graphs 420 and 440 for the effect of shotdosage, the slope of dosage curve 442 at x-coordinates “a” and “b” ishigher than the slope of dosage curve 422 at the same x-coordinates,indicating that improved edge slope can be obtained for a single shot byincreasing dosage, if dosages of other shots remain the same.

FIG. 4F illustrates an example of a dosage graph 450, illustrating thecase where the dosages of all shots have been increased to 150% ofnormal dose. Two background dosage levels are shown on dosage graph 450:a 30% background dose 459, such as may be produced if all shots use 150%of normal dosage, and a 20% background dose 458 shown for comparison,since 20% is the background dosage in the dosage graph 440. Dosage curve452 is based on the 30% background dose 459. As can be seen, the edgeslope of dosage curve 452 at x-coordinates “a” and “b” is less than thatof dosage curve 442 at the same points.

In summary, FIGS. 4A-F illustrate that higher-than-normal dosage can beused selectively to lower CD variation for isolated shapes. Increasingdosage has two undesirable effects, however. First, an increase in doseis achieved in modern charged particle beam writers by lengtheningexposure time. Thus, an increase in dose increases the writing time,which increases cost. Second, as illustrated in FIGS. 4E-F, if manyshots within the backscatter range of each other use an increaseddosage, the increase in backscatter reduces the edge slope of all shots,thereby worsening CD variation for all shots of a certain assigneddosage. The only way for any given shot to avert this problem is toincrease dosage and shoot a smaller size. However, doing this increasesthe backscatter even more. This cycle causes all shots to be at a higherdose, making write times even worse. Therefore, it is better to increasedose only for shots that define the edge.

FIG. 5A illustrates an example of a square VSB shot 502. In this examplesquare 502 has a dimension 504 of 100 nm. Pattern 506 is an example ofhow shot 502, with a normal dose, may register on a resist-coatedsurface. As can be seen, the corners 508 of pattern 506 are rounded, dueto beam blur. If formed on a reticle to be used for EUV opticallithography using 4× reduction printing, pattern 506 could be used toform a pattern on a wafer having a size of approximately 25 nm. FIG. 5Billustrates an example of a smaller square VSB shot 512. In thisexample, the dimension 514 of shot 512 is 60 nm, suitable formanufacturing a 4× reticle for a pattern intended to be 15 nm on awafer. Pattern 516 is an example of how shot 512 may register on aresist-coated surface. As can be seen, the corner rounding effects ofbeam blur have caused the registered pattern to be virtually circular.Additionally, though not illustrated, the edge slope of pattern 516 willbe lower than that of pattern 506, and may be below a minimumpre-determined level to produce acceptable CD variation. FIGS. 5A&Billustrate how beam blur effects become more significant as patterndimensions decrease.

As fabrication processes get smaller, short-range beam blur effectsbecome a more significant issue for both direct-write and forreticle/mask fabrication. Small geometries can also have problems withedge slope due to long-range effects. Accurate fabrication of the endsof minimum-width lines—that is the lines having the minimum widthpermissible in a fabrication process—can become challenging usingconventional techniques, as will be shown below. One type of pattern onwhich these problems may be exhibited is at a line end, which is theregion near an end of a path, where the path may be of constant width,such as interconnect lines or where polysilicon crosses and extendsbeyond diffusion to form a MOS transistor.

FIG. 6A illustrates an example of a portion 602 of a line that isdesired to be formed on a reticle. The portion includes line end 604. Inthis example the designed width on the wafer is 20 nm. Using a 4× mask,the target width 606 on the reticle is therefore 80 nm. FIG. 6Billustrates an example of an outline of a single VSB shot 614 that maybe used with normal dosage to conventionally form the pattern on areticle. FIG. 6B also illustrates a pattern 618 formed on the reticle bythe shot 614. As can be seen, the corners of line-end pattern 618 aresignificantly rounded. A portion 619 of the perimeter of pattern 618 isillustrated with a dashed line, indicating that this portion of theperimeter has an edge slope that is less than a pre-determined minimum.FIG. 6C illustrates an example of a method for forming the pattern 602according to the current invention. In FIG. 6C, two shots are used toexpose the line-end pattern 602: shot 624 and shot 625 which overlapsshot 624. Shot 624 uses higher-than-normal dosage. The additional shot625 provides additional peak dosage near the line end. Shot 625 uses alower dosage than shot 624, if assigned shot dosages are allowed. Ifassigned shot dosages are not allowed, multi-pass exposure may be usedwith shot 625 being grouped into an exposure pass having a lower basedosage than the exposure pass with shot 624. The two shots 624 and 625can produce a pattern 628 on the reticle, where the corners of pattern628 are less rounded than the corners of pattern 618. The dashed lineportions 629 of the perimeter of pattern 628 is shorter than the dashedline portion 619 of pattern 618, indicating improved line end edge slopein pattern 628, due to the higher line-end exposure in pattern 628compared to pattern 618.

FIG. 6D illustrates another embodiment of the current invention, usingthree shots to form the line end 604 of pattern 602. Shot 634 useshigher-than-normal dosage, like shot 624 of FIG. 6C. Additionally, shots635 and shot 636 overlap shot 634 and add additional peak dosage nearthe line end corners. Shots 635 and 636 may have lower dosage than shot634. Shots 635 and 636 may, as illustrated in this example, extendbeyond the outline of shot 634 and of the original pattern 602. Also,the illustrated shapes 635 and 636 may be shot as separate VSB shots, orin a single CP shot if a complex CP character is designed with the twoillustrated shapes 635 and 636. The three VSB shots 634, 635 and 636, ortwo shots if a CP shot is used to shoot illustrated shapes 635 and 636,can produce a pattern 638 on a reticle, where pattern 638 corners areless rounded than the corners of pattern 628 which resulted from twoshots. Additionally, low edge slope portion 639 of the perimeter ofpattern 638 is smaller than perimeter portion 629 of pattern 628. FIG.6D illustrates how larger numbers of shots may be used to form line endpatterns which both more accurately achieve the desired shape and whichhave a higher edge slope.

FIG. 6E illustrates yet another embodiment of the current invention,using four shots to form the line end 604 of pattern 602. In addition tomain shot 644, which may have a higher-than-normal dosage, two cornershots 645 and 646 are used, and shot 647 adds exposure to the middle ofthe line-end. The dosage of shot 647 may be less than the dosage ofshots 645 and 646. Shot 647 allows the dosage in the middle of the lineend to be adjusted independently of the dosages for the line endcorners. Pattern 648 illustrates a pattern that shots 644, 645, 646 and647 can produce on a reticle. In pattern 648 the perimeter portion 649which has a lower-than-minimum edge slope is slightly smaller than FIG.6D perimeter portion 639. Additionally, the illustrated shapes 645 and646 may be shot as a single complex CP character shot, if these shapesare designed and fabricated on a stencil.

FIGS. 6C-E illustrate how a set of shots may be modified withoverlapping shots to produce small areas of high peak dosage near lineends, improving both the accuracy and the edge slope of the patternmanufactured on a reticle. Exposure of only a small area with ahigher-than-normal dosage produces less increase in backscatter than ifthe higher-than-normal dosage was used for the entire pattern. The shotsare modified with a shot varying technique, which may include varyingone or all of: shot dosages, the placement of the overlap, and the sizeof the overlapping shot. Particle beam simulation may be used todetermine the effect that a set of shots and dosages will produce on thereticle surface.

FIGS. 9A-E illustrate the use of overlapping shots with square patterns,such as are commonly used for contact and via patterns in integratedcircuit design. FIG. 9A illustrates an example of a desired pattern 902to be formed on a reticle. FIG. 9B illustrates a single VSB shot 912which may be used to form pattern 902 conventionally. For smallpatterns, however, use of single VSB shot 912 may cause corner roundingsimilar to the corner rounding illustrated in FIG. 6B pattern 618. Alsolike pattern 618, use of single shot 912 may cause edge slope to beundesirably low. FIG. 9C illustrates an example of one embodiment of thepresent invention for forming a square or nearly-square pattern. FiveVSB shots may be used, including shot 922, which is cross-hatched foridentification, and four VSB corner shots 924 which overlap the cornersof shot 922. Alternatively, all four illustrated corner shapes 924 maybe designed into a single complex CP character on a stencil, allowingthe example of FIG. 9C to be shot with one VSB shot 922 and one CP shot924. As with the FIG. 6D line-end shot configuration, the addition ofcorner shots to increase peak dosage near the corners of the pattern mayimprove the fidelity of the transferred pattern, and may also improvethe edge slope near the corners of the transferred pattern, so as toreduce CD variation.

FIG. 9D illustrates an example of another embodiment of the presentinvention. Like the FIG. 9C shot configuration, FIG. 9D may be shotusing five VSB shots, including shot 932, which is cross-hatched, andfour additional shots 934 around the perimeter areas of the originalpattern 902. Also like FIG. 9C, a CP character may be designed to exposethe pattern illustrated by the four rectangles 934 in a single CP shot,allowing FIG. 9D to be exposed in one VSB shot 932 and one CP shot forall shapes 934. The use of the perimeter CP shot or VSB shots canincrease the edge slope of the entire perimeter of the transferredpattern by increasing peak dosage near the perimeter. The smallperimeter CP shot or VSB shots do not increase the area dosage as muchas if a higher dosage was used for shot 912, reducing the backscattercompared to if a higher dosage shot 912 was used alone.

FIG. 9E illustrates an example of another embodiment of the presentinvention. Nine regions are illustrated in FIG. 9E: a) a large region942, b) four side regions 944, and c) four corner regions 948. As can beseen, all regions 944 and 948 overlap region 942. These regions may beexposed by any of the following methods:

-   -   Nine separate VSB shots, including one for region 942, four        shots for the four regions 944, and four shots for the four        corner regions 948.    -   Five VSB shots. Region 942 is exposed by one shot. For the        remaining four VSB shots, each shot includes the union of one        side region 944 and two corner regions 948 adjacent to the side        regions. This provides a higher dosage at the corners than along        the side perimeters. The additional peak exposure near the        corner may provide improved accuracy and/or edge slope.    -   One VSB shot for region 942 and two CP shots—one shot each of        two CP characters. One CP character may be designed, for example        to include the four side regions 944 and a second CP character        may be designed to include the four corner regions 948. This        solution allows independent dosage control of the corner regions        and non-corner side regions.        The method using one VSB shot with two CP shots should require        less exposure time than either the nine-shot VSB or the        five-shot VSB methods. Additionally, the size of shot 942 may be        modified to be smaller than the desired pattern 902.

The methods of this invention may also be employed with fabricationprocesses that use rectangular contacts and/or vias. For rectangularpatterns with an aspect ratio of about 1:1.5 or less, the methodillustrated in FIG. 9D may be used. For rectangular patterns withgreater aspect ratios, each end of the longer axes of the rectangularpattern may be treated as a line end.

The solution described above with FIG. 9C may be implemented even usinga charged particle beam system that does not allow dosage assignment forindividual shots. In one embodiment of the present invention, a smallnumber of dosages may be selected, for example two dosages such as 1.0×normal and 0.6× normal, and shots for each of these two dosages may beseparated and exposed in two separate exposures passes, where the basedosage for one exposure pass is 1.0× normal and the base dosage for theother exposure pass is 0.6× normal. In the example of FIG. 9C, shot 922may be assigned to a first exposure pass which uses a base dosage of1.0× normal dosage before PEC correction. The four shots 924 may beassigned to a second exposure pass which uses a base dosage of 0.6×normal dosage before PEC correction. Thus, overlapping shots can createpattern dosages greater than 100% of normal, even with charged particlebeam writers which do not support dosage assignment for individualshots.

The dosage that would be received by a surface can be calculated andstored as a two-dimensional (X and Y) dosage map called a glyph. Atwo-dimensional dosage map or glyph is a two-dimensional grid ofcalculated dosage values for the vicinity of the shots comprising theglyph. This dosage map or glyph can be stored in a library of glyphs.The glyph library can be used as input during fracturing of the patternsin a design. For example, referring again to FIG. 9D, a dosage map maybe calculated for the combination of shots 932 and the four shots 934and stored in the glyph library. If during fracturing, one of the inputpatterns is a square pattern of the same size as pattern 902, the glyphfor pattern 902 and the five shots comprising the glyph may be retrievedfrom the library, avoiding the computational effort of determining anappropriate set of shots to form the square input pattern. Glyphs mayalso contain CP shots, and may contain dragged CP or VSB shots. A seriesof glyphs may also be combined to create a parameterized glyph.Parameters may be discrete or may be continuous. For example, the shotsand dosage maps for forming square patterns such as square pattern 902may be calculated for a plurality of pattern sizes, and the plurality ofresulting glyphs may be combined to form a discrete parameterized glyph.In another example, a pattern width may be parameterized as a functionof dragged shot velocity.

FIG. 7 is a conceptual flow diagram 750 of how to prepare a reticle foruse in fabricating a surface such as an integrated circuit on a siliconwafer. In a first step 752, a physical design, such as a physical designof an integrated circuit, is designed. This can include determining thelogic gates, transistors, metal layers, and other items that arerequired to be found in a physical design such as that in an integratedcircuit. Next, in a step 754, optical proximity correction isdetermined. In an embodiment of this disclosure this can include takingas input a library of pre-calculated glyphs or parameterized glyphs 776.This can also alternatively, or in addition, include taking as input alibrary of pre-designed characters 770 including complex characters thatare to be available on a stencil 760 in a step 762. In an embodiment ofthis disclosure, an OPC step 754 may also include simultaneousoptimization of shot count or write times, and may also include afracturing operation, a shot placement operation, a dose assignmentoperation, or may also include a shot sequence optimization operation,or other mask data preparation operations, with some or all of theseoperations being simultaneous or combined in a single step. Once opticalproximity correction is completed a mask design is developed in a step756.

In a step 758, a mask data preparation operation which may include afracturing operation, a shot placement operation, a dose assignmentoperation, or a shot sequence optimization may take place. Either of thesteps of the OPC step 754 or of the MDP step 758, or a separate programindependent of these two steps 754 or 758 can include a program fordetermining a limited number of stencil characters that need to bepresent on a stencil or a large number of glyphs or parameterized glyphsthat can be shot on the surface with a small number of shots bycombining characters that need to be present on a stencil with varyingdose, position, and degree of partial exposure to write all or a largepart of the required patterns on a reticle. Combining OPC and any or allof the various operations of mask data preparation in one step iscontemplated in this disclosure. Mask data preparation step 758, whichmay include a fracturing operation, may also comprise a pattern matchingoperation to match glyphs to create a mask that matches closely to themask design. In some embodiments of this disclosure, mask datapreparation step 758 may include generating overlapping shots so as toproduce a higher peak dosage near line ends or near perimeters of squareor nearly-square patterns. Mask data preparation may also compriseinputting patterns to be formed on a surface with the patterns beingslightly different, selecting a set of characters to be used to form thenumber of patterns, the set of characters fitting on a stencil mask, theset of characters possibly including both complex and VSB characters,and the set of characters based on varying character dose or varyingcharacter position or applying partial exposure of a character withinthe set of characters or dragging a character to reduce the shot countor total write time. A set of slightly different patterns on the surfacemay be designed to produce substantially the same pattern on asubstrate. Also, the set of characters may be selected from apredetermined set of characters. In one embodiment of this disclosure, aset of characters available on a stencil in a step 770 that may beselected quickly during the mask writing step 762 may be prepared for aspecific mask design. In that embodiment, once the mask data preparationstep 758 is completed, a stencil is prepared in a step 760. In anotherembodiment of this disclosure, a stencil is prepared in the step 760prior to or simultaneous with the MDP step 758 and may be independent ofthe particular mask design. In this embodiment, the characters availablein the step 770 and the stencil layout are designed in step 772 tooutput generically for many potential mask designs 756 to incorporateslightly different patterns that are likely to be output by a particularOPC program 754 or a particular MDP program 758 or particular types ofdesigns that characterizes the physical design 752 such as memories,flash memories, system on chip designs, or particular process technologybeing designed to in physical design 752, or a particular cell libraryused in physical design 752, or any other common characteristics thatmay form different sets of slightly different patterns in mask design756. The stencil can include a set of characters, such as a limitednumber of characters that was determined in the step 758, including aset of adjustment characters.

Once the stencil is completed the stencil is used to generate a surfacein a mask writer machine, such as an electron beam writer system. Thisparticular step is identified as a step 762. The electron beam writersystem projects a beam of electrons through the stencil onto a surfaceto form patterns in a surface, as shown in a step 764. The completedsurface may then be used in an optical lithography machine, which isshown in a step 766. Finally, in a step 768, a substrate such as asilicon wafer is produced. As has been previously described, in step 770characters may be provided to the OPC step 754 or the MDP step 758. Thestep 770 also provides characters to a character and stencil design step772 or a glyph generation step 774. The character and stencil designstep 772 provides input to the stencil step 760 and to the charactersstep 770. The glyph generation step 774 provides information to a glyphsor parameterized glyphs step 776. Also, as has been discussed, theglyphs or parameterized glyphs step 776 provides information to the OPCstep 754 or the MDP step 758.

Referring now to FIG. 8, another exemplary conceptual flow diagram 800of how to prepare a surface which is directly written on a substratesuch as a silicon wafer is shown. In a first step 802, a physicaldesign, such as a physical design of an integrated circuit is designed.This may be an ideal pattern that the designer wants transferred onto asubstrate. Next, in a step 804, various data preparation (DP) steps,including fracturing and PEC, are performed to prepare input data to asubstrate writing device. Step 804 may include fracturing of thepatterns into a set of complex CP and/or VSB shots, where some of theshots may overlap each other. The step 804 may also comprise inputtingpossible glyphs or parameterized glyphs from step 824, the glyphs beingbased on predetermined characters from step 818, and the glyphs beingdetermined using a calculation of varying a character dose or varying acharacter position or applying partial exposure of a character in glyphgeneration step 822. The step 804 may also comprise pattern matching tomatch glyphs to create a wafer image that matches closely to thephysical design created in the step 802. Iterations, potentiallyincluding only one iteration where a correct-by-construction“deterministic” calculation is performed, of pattern matching, doseassignment, and equivalence checking may also be performed. In someembodiments of this disclosure, data preparation step 804 may includegenerating overlapping shots near the line ends or near the perimetersof square or nearly-square patterns. A stencil is prepared in a step 808and is then provided to a wafer writer in a step 810. Once the stencilis completed the stencil is used to prepare a wafer in a wafer writermachine, such as an electron beam writer system. This step is identifiedas the step 810. The electron beam writer system projects a beam ofelectrons through the stencil onto a surface to form patterns in asurface. The surface is completed in a step 812.

Further, in a step 818 characters may be provided to the datapreparation and PEC step 804. The step 818 also provides characters to aglyph generation step 822. The character and stencil design step 820provides input to the stencil step 808 or to a character step 818. Thecharacter step 818 may provide input to the character and stencil designstep 820. The glyph generation step 822 provides information to a glyphsor parameterized glyphs step 824. The glyphs or parameterized glyphsstep 824 provides information to the Data Prep and PEC step 804. Thestep 810 may include repeated application as required for each layer ofprocessing, potentially with some processed using the methods describedin association with FIG. 7, and others processed using the methodsoutlined above with respect to FIG. 8, or others produced using anyother wafer writing method to produce integrated circuits on the siliconwafer.

The fracturing, mask data preparation, proximity effect correction andglyph creation flows described in this disclosure may be implementedusing general-purpose computers with appropriate computer software ascomputation devices. Due to the large amount of calculations required,multiple computers or processor cores may also be used in parallel. Inone embodiment, the computations may be subdivided into a plurality of2-dimensional geometric regions for one or more computation-intensivesteps in the flow, to support parallel processing. In anotherembodiment, a special-purpose hardware device, either used singly or inmultiples, may be used to perform the computations of one or more stepswith greater speed than using general-purpose computers or processorcores. In one embodiment, the special-purpose hardware device may be agraphics processing unit (GPU). In another embodiment, the optimizationand simulation processes described in this disclosure may includeiterative processes of revising and recalculating possible solutions, soas to minimize either the total number of shots, or the total chargedparticle beam writing time, or some other parameter. In yet anotherembodiment, an initial set of shots may be determined in acorrect-by-construction method, so that no shot modifications arerequired.

While the specification has been described in detail with respect tospecific embodiments, it will be appreciated that those skilled in theart, upon attaining an understanding of the foregoing, may readilyconceive of alterations to, variations of, and equivalents to theseembodiments. These and other modifications and variations to the presentmethods for fracturing, mask data preparation, and proximity effectcorrection may be practiced by those of ordinary skill in the art,without departing from the spirit and scope of the present subjectmatter, which is more particularly set forth in the appended claims.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tobe limiting. Steps can be added to, taken from or modified from thesteps in this specification without deviating from the scope of theinvention. In general, any flowcharts presented are only intended toindicate one possible sequence of basic operations to achieve afunction, and many variations are possible. Thus, it is intended thatthe present subject matter covers such modifications and variations ascome within the scope of the appended claims and their equivalents.

What is claimed is:
 1. A method for fracturing or mask data preparationor proximity effect correction comprising: calculating a line endpattern that will be produced on a resist-coated surface from anoriginal set of shots for a shaped beam charged particle beam writer;and modifying the original set of shots to improve the accuracy of thecalculated line end pattern near the line end, wherein the modificationcomprises at least one of the group consisting of 1) determining anadditional shot which overlaps a shot in the set of shots; 2) varyingthe overlap of two or more shots in the set of shots; 3) varying thesize of a shot which overlaps another shot; and 4) varying the dosage ofa shot in the set of shots with respect to the dosage of anotheroverlapping shot in the set of shots, and wherein the calculating andthe modifying are performed using one or more computing hardwareprocessors.
 2. The method of claim 1 wherein the calculating comprisescharged particle beam simulation.
 3. The method of claim 2 wherein thecharged particle beam simulation includes at least one of a groupconsisting of forward scattering, backward scattering, resist diffusion,Coulomb effect, etching, fogging, loading and resist charging.
 4. Themethod of claim 1 wherein the modified set of shots includes a characterprojection shot of a complex character.
 5. The method of claim 1 whereinthe modified set of shots increases the peak dosage near the line end.6. A method for fracturing or mask data preparation or proximity effectcorrection comprising: determining a plurality of shots for a shapedbeam charged particle beam writer, wherein the plurality of shots formsa line end pattern on a resist-coated surface, wherein the determiningcomprises calculating the pattern on the surface from the plurality ofshots, wherein the accuracy of the line end pattern on the surface isimproved using a shot varying technique comprising at least one of thegroup consisting of 1) varying the dosage of a shot overlapping anothershot; 2) varying the overlap of two or more shots; and 3) varying thesize of a shot which overlaps another shot, and wherein the determiningis performed using one or more computing hardware processors.
 7. Themethod of claim 6 wherein the determining comprises determining shotsfor multiple exposure passes, and wherein overlapping shots are placedin different exposure passes.
 8. The method of claim 6 wherein a complexcharacter shot is determined.
 9. The method of claim 6 wherein anoptimization technique is used to determine the plurality of shots. 10.The method of claim 6 wherein the calculating comprises charged particlebeam simulation.
 11. The method of claim 6, further comprising inputtinga library of precalculated glyphs, wherein the determining determinesshots from one or more glyphs, and wherein the glyph precalculationconstitutes at least a part of the pattern calculation.
 12. A method forfracturing or mask data preparation or proximity effect correctioncomprising: determining a plurality of shots for a shaped beam chargedparticle beam writer, wherein the plurality of shots forms a square ornearly-square pattern on a resist-coated surface, wherein thedetermining comprises calculating the pattern on the surface from theplurality of shots, wherein the accuracy of the square pattern on thesurface is improved using a shot varying technique comprising at leastone of the group consisting of 1) varying the dosage of a shotoverlapping another shot; 2) varying the overlap of two or more shots;and 3) varying the size of a shot which overlaps another shot, andwherein the determining is performed using one or more computinghardware processors.
 13. The method of claim 12 wherein the calculatingcomprises charged particle beam simulation.
 14. The method of claim 12wherein the plurality of shots produces a higher peak dosage near thecorners of the pattern than in the center of the pattern.
 15. A methodfor manufacturing a surface comprising: determining a plurality of shotsfor a shaped beam charged particle beam writer, wherein the plurality ofshots is capable of forming a line end pattern on a surface, wherein thedetermining comprises calculating the pattern on the surface from theplurality of shots, and wherein the accuracy of the line end pattern onthe surface is improved using a shot varying technique comprising atleast one of the group consisting of 1) varying the dosage of a shotoverlapping another shot; 2) varying the overlap of two or more shots;and 3) varying the size of a shot which overlaps another shot; andforming the line end pattern on the surface using the plurality ofcharged particle beam shots.
 16. The method of claim 15 wherein thedetermining comprises determining shots for multiple exposure passes,and wherein overlapping shots are placed in different exposure passes.17. The method of claim 15 wherein the set of shots includes a complexcharacter.
 18. The method of claim 15 wherein the calculating comprisescharged particle beam simulation.
 19. A method for manufacturing asurface comprising: determining a plurality of shots for a shaped beamcharged particle beam writer, wherein the plurality of shots is capableof forming a square or nearly-square pattern on a surface, wherein thedetermining comprises calculating the pattern on the surface from theplurality of shots, and wherein the accuracy of the pattern on thesurface is improved using a shot varying technique comprising at leastone of the group consisting of 1) varying the dosage of a shotoverlapping another shot; 2) varying the overlap of two or more shots;and 3) varying the size of a shot which overlaps another shot; andforming the square or nearly-square pattern on the surface using theplurality of charged particle beam shots.
 20. A system for fracturing ormask data preparation or proximity effect correction comprising: adevice capable of determining a plurality of charged particle beam shotswhich are capable of forming a line end pattern on a surface, whereinthe device capable of determining comprises a device capable ofcalculating the pattern on the surface from the plurality of shots, andwherein the accuracy of the line end pattern on the surface is improvedusing a shot varying technique comprising at least one of the groupconsisting of 1) varying the dosage of a shot overlapping another shot;2) varying the overlap of two or more shots; and 3) varying the size ofa shot which overlaps another shot.
 21. The system of claim 20 whereinthe device capable of calculating performs charged particle beamsimulation.
 22. The system of claim 21 wherein the charged particle beamsimulation includes at least one of a group consisting of forwardscattering, backward scattering, resist diffusion, Coulomb effect,etching, fogging, loading and resist charging.
 23. The system of claim20 wherein the device capable of determining uses an optimizationtechnique.
 24. A system for fracturing or mask data preparation orproximity effect correction comprising: a device capable of determininga plurality of charged particle beam shots which are capable of forminga square or nearly-square pattern on a surface, wherein the devicecapable of determining comprises a device capable of calculating thepattern on the surface from the plurality of shots, and wherein theaccuracy of the square pattern on the surface is improved using a shotvarying technique comprising at least one of the group consisting of 1)varying the dosage of a shot overlapping another shot; 2) varying theoverlap of two or more shots; and 3) varying the size of a shot whichoverlaps another shot.
 25. The system of claim 24 wherein the devicecapable of calculating performs charged particle beam simulation.